Method of making and joining an aerofoil and root

ABSTRACT

A ducted fan gas turbine engine aerofoil is made by electron beam welding together at least two metal sheets ( 10 ) and ( 12 ) and electron beam welding that sub assembly via an end to a root that has been manufactured in a separate operation, and then heating the whole to a temperature that will convert the electron beam welds to diffusion bonds.

The present invention relates to the manufacture of aerofoil blades of the kind used in ducted fan gas turbine engines, wherein the aerofoils are located via respective roots, in and about the rim of a rotary disk within a ducted fan gas turbine engine.

More specifically, the present invention has best efficacy where used in the manufacture of gas turbine engine fan blades, the aerofoils of which are hollow.

It is known to manufacture a hollow fan blade by forming two half aerofoils, one of which provides a concave exterior surface, and the other of which provides a convex exterior surface, and both include a half root portion. The formed halves are then placed in a die and heated so as to enable diffusion bonding of the halves and super-plastic expansion and separation in known manner of the interior surfaces of the joined aerofoils to cause movement of the aerofoils into their respective curved forms.

The present invention seeks to provide an improved method of making and joining a hollow aerofoil and root.

According to the present invention there is provided a method of making an at least substantially hollow aerofoil having a separately manufactured root comprises the steps of welding at least two metal sheets together about their edges, manufacturing a root having a surface shaped to receive an end of said joined sheets, welding said end to said surface, and then holding the resulting assembly in holding means via said sheets and heating the assembly to convert the weld joints to diffusion bonds.

The invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal cross section through an aerofoil on line 1-1 of FIG. 2.

FIG. 2 is a diagrammatic sketch of a ducted fan gas turbine engine including a stage of fan aerofoils in accordance with the present invention.

Referring to FIG. 1. two sheets of metal, 10 and 12, which may be titanium or aluminium, are welded together around their edges 14. A third, much thinner metal sheet 16 of the same material as sheets 10 and 12, is trapped between sheets 10 and 12, and is further fixed by the weld referred to hereinbefore. A root member 18 that has been manufactured separate from the sheets 10, 12 and 16, is provided with a surface 20 to which, via an end, the assembly of sheets 10, 12 and 16 is fixed by e.g. electron beam welding, or linear friction welding. Root member 18 is so shaped as to be a sliding fit in a respective groove in the rim of a fan disk 22 of engine 24 in FIG. 2.

All of the parts making up the assembly are of a common material e.g. titanium or aluminium, and in the present example of the invention, it is intended that they be diffusion bonded after the welding operation. However, where thin plate 16 is used, lengthwise strip portions thereof are later required to stretch in opposing directions laterally of the sheet length, so as to provide a stiffening member for the aerofoil.

Therefore, a number of strips of a diffusion bond preventative such as Yttria are glued on to each side of sheet 16 prior to its insertion between sheets 10 and 12.

When the assembly is completed as described so far, it is placed in a suitable die which will enable forming sheets 10 and 12 into an aerofoil shape, and subjected to heat and temperature, the magnitudes of both of which are well known in the diffusion bonding and super-plastic forming field. Piping is connected to the interior of the sheets and an inert gas pumped in so as to cause sheets 10 and 12 to move away from each other to form the aerofoil shape dictated by the die, and simultaneously pull spaced portions of sheet 16 in opposing directions, to form the stiffening member. Also effected is the conversion of all of the welded joints peripherally of the sheets and between the ends of the sheets and root 18 to diffusion bonds, wherein material from each part migrates across the joint interface and eliminates it.

Attaching the root 18 to sheets 10, 12 and 16 at the stage in the process described provides the advantage that the following diffusion bonding process relieves stresses that are generated in the joint area during welding, thus obviating the need to perform a separate operation to achieve that effect. Further, it has been found that the resulting strength of the finished article is such that thinner sheets may be used without detriment.

Exclusion of sheet 16 will enable the manufacture of a completely hollow aerofoil having a root attached in the manner as described with reference to FIG. 1. In this example, that surface on one of the sheets that will be an interior surface when the two sheets are assembled, will have yttria applied to that area not required to diffusion bond.

An alternative method of manufacturing an aerofoil blade and root, is to weld sheets 10 and 12, or sheets 10, 12 and 16 together as described hereinbefore, and then super-plastically form them into the desired aerofoil shape, prior to welding them to root 18. The finished aerofoil can then be welded to root 18. The whole will then be heated to achieve conversion of the root weld to a diffusion bond, again as described hereinbefore.

A further alternative method of manufacturing an aerofoil blade and root, is to weld sheets 10 and 12, or weld sheets 10 and 12 and 16, together as described hereinbefore, and then to diffusion bond them together, prior to welding them to the root 18. The sheets 10 and 12, or the sheets 10, 12 and 16 are then super-plastically formed into the desired aerofoil shape. The heating used by the super-plastic forming process relieves stresses in the joint area during welding and to form a diffusion bond. 

1. A method of making an at least substantially hollow aerofoil having a separately manufactured root comprising the steps of providing at least two metal sheets having edges, welding the at least two metal sheets together about their edges, manufacturing a root having a surface shaped to receive an end of said joined metal sheets, welding said end of the joined metal sheets to said surface of the root to form an assembly, and then holding the assembly in holding means via said metal sheets, and heating the assembly to convert the weld joints to diffusion bonds.
 2. A method of making an at least substantially hollow aerofoil as claimed in claim 1 wherein only two metal sheets are used in the making of said aerofoil, at least one of which has a diffusion bond preventing material applied all over that surface which on assembly of the two metal sheets defines an interior surface of said assembly.
 3. A method of making an at least substantially hollow aerofoil as claimed in claim 1 wherein three metal sheets are used in the making of said aerofoil, the central metal sheet being thinner than the outer metal sheets, the central sheet having strips of diffusion bond preventing material applied to both sides thereof, the positions of said strips on one side of said central metal sheet being staggered with respect to the positions of the strips on the other side of the central metal sheet.
 4. A method of making an at least substantially hollow aerofoil as claimed in claim 1 including the use of holding means comprising a die, the die having inner opposing surfaces, the inner opposing surfaces having respective aerofoil suction and pressure forms, and on achievement of said diffusion bonding heating temperature, pumping an inert gas between the metal sheets via piping so as to expand them against the respective inner opposing surfaces of said die so as to cause them to adopt a corresponding aerofoil form.
 5. A method of making an at least substantially hollow aerofoil as claimed in claim 3 including the use of holding means comprising a die, the die having inner opposing surfaces, the inner opposing surfaces having respective aerofoil suction and pressure forms, and on achievement of said diffusion bonding temperature, pumping an inert gas between each outer metal sheet and the respective opposing sides of said central metal sheet so as to expand said outer metal sheets against respective opposing inner surfaces of said die and cause them to adopt a corresponding aerofoil form, and to cause portions of said central metal sheet to super-plastically extend in staggered manner so as to form an aerofoil stiffening structure.
 6. A method of making an at least substantially hollow aerofoil as claimed in claim 1 including the step of placing the welded metal sheets assembly in a die having inner opposed respective aerofoil suction and pressure surfaces, heating the metal sheets and subjecting the interior thereof to a pressure sufficient to expand them into the respective die inner surfaces so as to adopt the aerofoil form, and then welding the formed aerofoil to said root and converting said weld joint to a diffusion bond.
 7. A method of making an at least substantially hollow aerofoil as claimed in claim 1 including the step of diffusion bonding the welded metal sheets assembly together, welding the diffusion bonded metal sheets to said root, placing the diffusion bonded metal sheets and root assembly in a die having inner opposed respective aerofoil suction and pressure surfaces, heating the metal sheets and subjecting the interior thereof to a pressure sufficient to expand them into the respective die inner surfaces so as to adopt the aerofoil form.
 8. A method of making an at least substantially hollow aerofoil as claimed in claim 1 wherein the welding of said end of the joined metal sheets to said surface of the root comprises election beam welding and friction welding. 